Nickel et al. 2006

Determination of Corrosion Layers and Protective Coatings on Steels and Alloys Used in

Simulated Service Environment of Modern Power Plants

Hubertus Nickel 1

e-mail: h.nickel@fz-juelich.de

Willem J. Quadakkers

Lorenz Singheiser

Institute for Materials and

Processes in Energy Systems,

Research Centre Jülich GmbH,

D-52056 Jülich, Germany,

and University of Technology,

Aachen, Germany

The development of modern power generation systems with higher thermal efficiency requires the use of constructional materials of higher strength and improved resistance to the aggressive service atmospheres. In this paper, the following examples are discussed. (i) The oxidation behavior of 9% Cr steels in simulated combustion gases: The effects of O 2 and H 2 O content on the oxidation behavior of 9% Cr steels in the temperature range 600–800°C showed that in dry oxygen a protective scale was formed with an oxidation rate controlled by diffusion. In contrast, that in the presence of water vapor, after an incubation period, the scale became nonprotective as a result of a change in the oxidation mechanism. (ii) The development of NiCrAlY alloys for corrosion-resistant coatings and

thermal barrier coatings of gas turbine components: The increase of component surface temperature in modern gas turbines leads to an enhanced oxidation attack of the blade coating. Considerable efforts have been made in the improvement of the temperature properties of MCrAlY coatings by the additions of minor elements, such as yttrium, silicon, and titanium. The experimental results show the positive, but different influence of the oxidation behavior of the MCrAlY coatings by the addition of these minor elements. (iii) The development of lightweight intermetallics of TiAl-basis: TiAl-based intermetal-lics are promising materials for future turbine components because of the combination of

high-temperature strength and low density. These alloys, however, possess poor oxidation resistance at temperatures above 700° C. The experimental results showed that the oxidation behavior of TiAl-based intermetallics can be strongly improved by minor additions of 1–2 at. % silver. (iv) The oxide-dispersion-strengthened (ODS) alloys provide excellent creep resistance up to much higher temperatures than can be achieved with conventional wrought or cast alloys in combination with suitable high-temperature oxidation/ corrosion resistance. The growth mechanisms of protective chromia and alumina scales were examined by a two-stage oxidation method with 18 O tracer. The distribution of the oxygen isotopes in the oxide scale was determined by secondary ion-mass spectroscopy and SNMS. The results show the positive influence of a Y 2 O 3 dispersion on the oxidation resistance of the ODS alloys and its effect on growth mechanisms.